There is heated debate in the United States about the extent to which waterborne infectious diseases may be transmitted to human beings through drinking water that meets federal standards for pathogen removal. One important issue in the debate about such transmission in endemic settings centers on concern that specific subpopulations (such as the elderly, young children, and immunocompromised individuals) may be at increased risk for both initial infection and serious sequelae. Studies have suggested that elderly individuals are at significant risk of infection from waterborne pathogens, and that they develop more severe disease. The principal objective of this proposal is to evaluate the ability of in-home treatment of tapwater to reduce gastrointestinal illness in non-institutionalized elderly individuals. This objective will be achieved through the use of a randomized, controlled, and double-blinded intervention trial among elderly persons living in Sonoma, California, who are already participants in an NIH-funded longitudinal cohort ("The Epidemiology of Aging and Physical Function", 2 R01 AG09389-06). A total of 540 households (810 individuals) are to be enrolled for 12 months each during this five-year study. A cross-over design will be used in which half the households will be randomly selected to receive treatment (a sealed water treatment device consisting of an ultraviolet lamp and a 1 micron filter) for six months and will be switched to a placebo device for the remaining six months of their study participation. The other households will initially receive the placebo for six months and end with the treatment device. Stool specimens will be collected and analyzed from individuals who report gastrointestinal illnesses during the study and tested for a broad range of bacterial, viral, and protozoal pathogens by the California Department of Health Services. The water quality will be systematically monitored to document that the water supply remained in compliance with applicable standards. This research will provide the following estimates among elderly individuals: 1) the attributable risk of gastrointestinal illness from the consumption of tap water derived from a surface water source that is occasionally contaminated with human waste (a widespread occurrence in the United States) but is treated to meet federal standards and 2) the rate of detection of specific bacterial, viral, and protozoal agents in stool specimens of elderly persons with gastrointestinal illness. The team submitting this proposal is currently conducting work, funded by the Centers for Disease Control and Prevention and the US Environmental Protection Agency, to improve subject blinding in randomized, blinded water intervention trials in individuals of all ages.